The present invention relates to blends of polypropylene based polymeric materials which are visbroken in order to impart to them both high impact and high melt flow rate properties. Impact strength properties are strongly dependent upon the melt flow rate of the polymer. Polymers which have a low (e.g., fractional) melt flow rate and are processable only with difficulty inherently have better impact resistance than the easily processed higher melt flow rate polymers. The combination of high melt flow rate and high impact resistance has heretofore not been obtained.
It is well-known in the art to blend polypropylene homopolymer with low or high density polyethylene and ethylene-propylene rubber in order to increase the impact resistance of the polymer without too great a sacrifice in stiffness. U.S. Pat. No. 4,088,714 discloses that the impact properties of polypropylene and cross-linkable low density polyethylene blends could be significantly improved by the addition of an ethylene-propylene rubber to the blend, and then curing the rubber with the cross-linkable polyethylene to form a discontinuous, tightly cross-linked phase of rubber and polyethylene which is intimately dispersed in a continuous phase of polypropylene. An agent is incorporated to prevent degradation of the polypropylene. The total blend assumed a beneficial melt flowable characteristic. The stiffness and tensile strength of this blend is comparatively low. The flexural moduli (1 percent secant) were only in the range of 20,000-36,000 psi (140-250 MPa) and the tensile strengths at yield were under 1500 psi (10 MPa). For the blends of the present invention, the flexural moduli are generally about 90,000 psi (600 MPa) and the tensile strengths are generally about 2900 psi (20 MPa). In U.S. Pat. No. 4,087,485 it was disclosed that an impact polypropylene blend having surprisingly good optical properties could be formulated by partially curing an ethylene-propylene rubber with a cross-linkable low density polyethylene in the presence of polypropylene. U.S. Pat. No. 4,221,882 discloses a melt-flowable, high impact stength melt mixed blend of polypropylene, ethylene-propylene rubber, and polyethylene wherein the ratio of the polypropylene to the combination of the other components is from about 1 to about 2, the polyethylene component is at least about 30 percent of the total blend, and the rubber component is from 4 to 11 percent of the total blend. In none of the above patents were the inventors working with the components of the present invention nor do they disclose visbreaking of the components after blending and the advantages thereof.
Visbreaking of polymers, a process for the controlled degradation thereof, is well-known. U.S. Pat. No. 3,144,436 discloses such a process for preparing low or moderate molecular weight stereoregular polymers which are more easily processable than stereoregular polymers of very high molecular weight. The high molecular weight stereoregular polymers are treated in the absence of oxygen at a temperature above the melting point of the polymer with a small amount of a free radical initiator to increase the melt flow rate of the polymers. U.S. Pat. No. 3,940,739 discloses a method for the degradation of propylene polymers to increase their melt flow rate which comprises contacting a propylene polymer with oxygen and an organic or inorganic peroxide, melting and working the resulting mixture in a high shear zone, and recovering an essentially odor-free propylene polymer. U.S. Pat. No. 4,061,694 discloses the manufacture of polypropylene molding compositions of improved impact strength by subjecting block copolymers of ethylene and propylene to controlled oxidative degradation under conditions essentially similar to those of the preceding patent. U.S. Pat. No. 3,923,947 discloses a process for preparing cross-linkable polyethylene compositions by contacting the compositions with a peroxy compound at a temperature above the melting point of the composition. The polyethylene composition can be a blend of high and low density polyethylene, low density polyethylene and ethylene vinyl acetate, high density polyethylene and ethylene vinyl acetate, and blends of the above with polypropylene. None of the above patents discloses the advantages of blending the two components of the composition of the present invention and then visbreaking them.